why instruct the engine above position is a draw, when SF finds mate very quickly even at bullet?

I'm not sure what point you think you're making?
The position is apparently mate in 43 moves (starting with 1. Bb3+), so the 50 move rule is irrelevant even if the knight is not captured (which I haven't checked). Source: Nalimov tables from http://www.k4it.de/?topic=egtb&lang=en.
If you're saying adjudicating a game deprives the engine of the ability to mishandle a position, or swindle it's opponent, sure. Different discussion though.

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score rises from 5 pawns to 48 pawns very quickly, SF has seen mate.

So why isn't it reporting a mate-score, then?

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what if SF has pruned above position, because its counter says 50-moves threshold, draw, to the benefit of this one

The position is not a 50 move draw, so if it had done that there's a bug to fix.
You also seem to confuse "pruning" with returning the known evaluation (and stopping the search) in tablebase positions.

I specifically said 'during the search', but you seem not to read.

You don't understand.
As soon as you capture into a 5-man position you access the tablebase. The capture, of course, reset the 50-move counter. The situation you scetch never occurs because the search never probes the tablebase excpt when the 50-move counter is 0.

rigth, I missed the fact that, in order to arrive at a tbs position, the last move should be a capture.

essentially this changes nothing though, you just have to use tbs mates that go over the 50-move counter.

looking at TCEC, SF shows frequently more than 30 000 000 tbs hits, if 1% of those are so called "cursed wins", this makes 300 000 cursed win hits. Imagine the impact on search decisions, gameplay and picking the best move.